Distributions and sources of low-molecular-weight monocarboxylic acids in gas and particles from a deciduous broadleaf forest in northern Japan

Abstract. To better understand the distributions and sources of low-molecular-weight (LMW) monocarboxylic acids (monoacids) in the forest atmosphere, we conducted simultaneous collection of gaseous and particulate samples at a deciduous broadleaf forest site in northern Japan. LMW normal (C1–C10), branched (iC4–iC6), hydroxyl (glycolic and lactic) and aromatic (benzoic) monoacids were detected in the gas and particle phases. The dominant LMW monoacids in gas phase were formic (mean: 953 ng m−3) and acetic (528 ng m−3) acids followed by propionic (37 ng m−3) or isopentanoic (42 ng m−3) acid. In the particle phase, isopentanoic (159 ng m−3) was dominant, followed by acetic (104 ng m−3) and formic (71 ng m−3) or lactic (65 ng m−3) acids. Concentrations of LMW monoacids did not show correlations with anthropogenic tracers such as nss-SO42- and NO3-, indicating that anthropogenic contribution is not important. Concentrations of C1–C6 monoacids in the gas phase showed positive correlations (r2=0.21–0.91) with isobutyric acid (iC4), which may be produced by microbial activity in soil. The forest soil may be a source of gaseous C1–C6 monoacids in the forest atmosphere. Acetic acid in the particle phase positively correlated with nonanoic acid (C9) (r2=0.63), suggesting that formation of acetic and nonanoic acids is associated with the oxidation of biogenic unsaturated fatty acids in the aerosol phase, in addition to photochemical oxidation of biogenic volatile organic compounds. The particle-phase fractions (Fp) of formic and acetic acids showed negative correlation with ambient temperature (C1: r2=0.49, C2: r2=0.60) but showed positive correlation with relative humidity (C1: r2=0.30, C2: r2=0.55) in daytime, suggesting that these meteorological parameters are important for the gas and particle portioning of monoacids in the forest atmosphere.